The present invention relates to an apparatus for optically measuring the distance from a subject vehicle, on which the apparatus is mounted, to objects such as a preceding vehicle running on the same lane of a road, or obstacles such as other vehicles around the preceding vehicle which are running on other lanes of the road or coming into the same lane at locations between the preceding vehicle and the subject vehicle.
Examples of an optical distance detecting apparatus utilizing image sensors such as video cameras are known from Japanese Patent Publication Nos. 63-38085 and 63-46363. As illustrated in FIG. 6, the apparatuses disclosed therein commonly have a pair of first and second parallel optical systems having two convex lenses 101, 102 disposed in a horizontally aligned relation at a prescribed distance L away from each other. A pair of separate image sensors 103, 104 are horizontally disposed at focal points of the lenses 101, 102 at a distance f from the locations of corresponding lenses 101, 102, respectively, for generating respective image signals to a common signal processor 120. The signal processor 120 successively shifts the image signals from the image sensors 103, 104 and electrically superposes them one over the other, so that the distance R from the lenses 101, 102 to an object 121 is calculated based on the principle of triangulation using the following formula: EQU R=(f.times.L)/d
where d is a shift distance by which the image signals are moved to obtain the best match.
In addition, Japanese Patent Publication No. 60-33352 discloses a method of following an image of a preceding vehicle as sensed by an image sensor such as a video camera. According to this method, the driver of a vehicle needs to manually set and continuously adjust an image-following gate or window enclosing an object to be followed on a display screen while looking at the screen, so as not to lose sight of the image of the preceding vehicle.
With the distance detecting apparatus as described above, however, it is only possible to measure the distance to an object which lies in a direction of the axis of light which is projected from each video camera. Therefore, in cases where the apparatus is mounted on a vehicle for the purpose of measuring the distance to a preceding vehicle, it is necessary to properly move the video cameras so as to adjust the axis of light projected therefrom toward the preceding vehicle. Moreover, if there are many objects within the field of view of the image sensors, it is unclear to which object the distance from the subject vehicle is being detected. In particular, if other vehicles travelling around or approaching the target vehicle are coming into the fields of view of the video cameras, the driver cannot tell to which vehicle the distance is detected.